This invention relates to a radiation curable coating for use on various substrates, and more particularly to a coating for substrates which renders them receptive to thermal transfer images and provides the substrate with resistance to heat, moisture, abrasion, and solvents.
In the field of product labeling, thermal transfer printing has become a well-known means of non-impact printing. Thermal transfer printing of bar codes not only provides fast inventory and quick identification of products, but also provides an advantage over direct thermal printing in that thermal transfer printing uses a ribbon with the print head whereas direct thermal printing is dependent upon having heat-reactive chemicals present in the substrate.
However, in thermal transfer printing, the print quality of the images transferred from the ribbon to the substrate are dependent upon the receptivity of the substrate surface. Many grades of paper have rough surfaces which are unsuitable for use in thermal transfer printing, and may result in character formation in which there are voids or irregular edges. Other substrates, including some grades of paper, may also be unreceptive to thermal transfer images. Further, where substrates have previously been printed with inks, such printed surfaces may not be receptive to thermal transfer images.
Protective coatings for substrates are known in the art for direct thermal and electrographic printing. In addition, coatings have been developed for protection of thermally printed images against environmental factors such as moisture, abrasion, and solvents. For example, U.S. Pat. No. 4,886,774 to Doi teaches a protective overcoating for thermal paper which comprises a light stabilizer, an additive for promoting curing of the coating by ultraviolet radiation, and a second additive which serves as a an UV absorbing compound to provide resistance to fading.
Arbee et al, U.S. Pat. No. 4,591,887, relates to a solvent-resistant thermally printable material for the manufacture of labels comprising a protective layer of polymeric resin on top of a thermally imprintable color producing layer and an adhesive layer.
Although these references disclose protection of thermally printed images, the coatings are applied to thermal paper for use in direct thermal printing. Thermally imageable paper has already been coated with a heat-sensitive substance to allow it to react and form an image when exposed to a thermal printer. The protective coatings discussed above thus only serve as an overcoat for the images which are formed within the thermally imageable coating on the paper, and are not designed to render paper receptive to thermal transfer images.
Other protective coatings have been developed for use on a variety of substrates such as wood, metal, paper, glass, and ceramic materials. Friedlander et al, U.S. Pat. No. 4,130,708, relates to a protective coating composition which includes radiation curable compounds formed from the reaction of a siloxy-containing carbinol, a polyisocyanate, and a polyfunctional compound having hydroxy and acrylic functional groups. However, the coating is not directed to thermal transfer printing, and thus does not address the problems associated with the transfer of such images to a substrate surface; i.e., complete character formation and receptivity to thermal transfer images.
Accordingly, there is a need in the art for a coating for substrates such as paper which makes them receptive to thermal transfer imaging, provides good print quality of thermal transfer images formed thereon, and provides the substrate with resistance to heat, moisture, abrasion, and solvents.